The invention relates generally to a roll apparatus for treating webs of material and, more particularly, to an improved hydraulically supported roll.
A roll apparatus having an upper roll and a hydraulically supported bottom roll is disclosed in pending U.S. application Ser. No. 07/370,348, now U.S. Pat. No. 4,984,343, entitled Improved Sealing Element For A Hydrostatically Supported Roll, filed June 22, 1989 and assigned to the assignee of the present invention. In this application, the bottom roll includes a rotatable hollow cylinder, which forms the working roll circumference, and a stationary crosshead extending lengthwise through the cylinder to form a surrounding clearance space therewith. The hollow cylinder is radially displaceable, as a whole, in the active plane in a direction away from the crosshead. At least near each end of the hollow cylinder, a radial support element, which includes a piston/cylinder unit, abuts the inner circumference of the hollow cylinder. The piston and cylinder of the piston/cylinder unit are disengaged before the hollow cylinder contacts the upper roll of the roll apparatus. The cylinder of the piston/cylinder unit is provided with choke bores and the piston is received in the cylinder near the end of piston's dropping motion to displace the hydraulic fluid in the cylinder through the choke bores. This produces a damping effect in the last phase of the dropping motion. However, in the "active" lifting range, there is a dead travel segment in which damping does not occur.
The invention is applicable to rolls in which the ends of hollow cylinder are not fixedly supported on the crosshead, i.e., the hollow cylinder is not radially locked in position at its ends by the bearings that support the cylinder for rotation relative to the crosshead. Rather, the hollow cylinder of the rolls of the invention is radially displaceable in the active plane, as a whole, relative to the crosshead by a guide arrangement provided at the ends of the crosshead for rotatably supporting the hollow cylinder and allowing the desired radial movement. Such guide arrangements are well known in the art and rolls having such guide arrangements are commonly referred to as rolls having an "inner lift". One example of such a guide arrangement is specifically disclosed in FIG. 9 of U.S. Pat. No. 4,856,157.
An internal hydraulic support device provides for the pressing action of the hollow cylinder in the roll nip. The hydraulic support device may be provided in various ways, such as those described in DE-PS 10 26 609, DE-OS 22 30 139, U.S. Pat. No. 2,395,915, DE-PS 14 61 066, DE-OS 30 03 395 or U.S. Pat. No. 4,856,157.
In all rolls supported solely by a plurality of hydraulic force exerting elements, the problem of synchronous control, i.e. synchronizing the movements of the elements, exists. Care must be taken that the hollow cylinder is not positioned at a skew relative to the roll nip and that it does not abut at one end, while a large opening is present at the other end.
To solve this problem in rolls of the type in question, which have supporting elements distributed over the length of the roll functioning as an internal hydraulic support device, volume division by means of a pump providing the same volume of fluid to a plurality of outputs has been considered. With such a pump, the individual supporting elements are allotted essentially the same amount of hydraulic fluid. This type of arrangement demands a considerable degree of sophistication and, nevertheless, is not always satisfactory, because the hydraulic supporting elements may have different widths, so that even when the volumes are allotted equally, different displacements can occur.
Another possibility for rolls of the type in question would be a synchronous control in which the initial displacements of the supporting elements are measured at both ends of the roll and whenever one side advances more than the other, the quantity of hydraulic fluid fed to this side of the supporting elements is reduced. Even this type of control is costly.